October 15, 2026 is the end of the line — not another extension, the last one. If you filed for more time in April, this is the Thursday your 2025 return comes due. After it there are only penalties, counted backward to April.

For most La Crescenta and Glendale owners that means Form 1040, carrying the Schedule C or the K-1 from your S corp.

What actually falls on the 15th

The IRS fourth-quarter calendar and the FTB’s tables agree on four items:

The extension was never an extension to pay

Both agencies say so. Form 7004 “does not extend the time to pay any tax due.” The FTB: “Pay the amount you owe by April 15, 2026 to avoid penalties and interest.”

If you owe, the federal failure-to-pay penalty has run since April at 0.5% of the unpaid tax per month or partial month, capped at 25%. It falls to 0.25% under an approved payment plan and rises to 1% if you ignore a notice of intent to levy for ten days. California adds 5% of the unpaid tax plus 0.5% a month, up to 40 months. One break for C corps: pay 90% by the original due date and the late-payment penalty holds off if the rest clears by October 15.

Miss the date and the meter runs ten times faster

The failure-to-file penalty is 5% of the tax due per month or partial month, up to 25% — ten times the rate for merely not paying, charged from the April due date. When both apply the filing penalty absorbs the 0.5%, so it is 5% a month for five months, then the paying penalty alone.

There is also a floor. For returns due after December 31, 2025, one more than 60 days late owes a minimum of $525 or 100% of the underpayment, whichever is less — owe $400, file in January, and the penalty is the whole $400. California’s delinquent filing penalty is also 5% a month to 25%, with its own floor: on an individual balance of $540 or less, the lesser of $135 or 100%.

Hence the rule: file by the 15th even if you cannot pay. Not paying is the cheap mistake. Not filing is the expensive one.

One California date is not October 15

California C corporations get seven months, not six: Form 100 is due the 15th day of the 11th month — November 15, a Sunday in 2026, so the next-business-day rule moves it to Monday, November 16. Federal Form 1120 does not move with it. The mirror image of September, when California gave partnerships a month the IRS did not. Every date for the year sits in our 2026 California deadline calendar.

What this means for you

If your 2025 return is still open, there is no more time to ask for. The only question is whether it goes out by Thursday the 15th, whatever the balance line says. Filing is free; not filing is 5% a month or $525, whichever hurts more. If this is your first slip after three clean years, the IRS’s new automatic penalty relief may absorb it — but only on a filed return.

Before October 15

  • Confirm the April extension was accepted — don’t assume the software sent it
  • Get the last K-1s, 1099s and statements to your CPA this week, not the 14th
  • File even if the balance is unpaid, then request a payment plan — 0.25% beats 0.5%
  • C corps: check whether you cleared the 90% payment mark in April
  • California C corps: Form 100 has until November 16; federal Form 1120 does not
  • Made $89,000 or less in 2025? IRS Free File is open to extension filers through the 15th
  • Then November 2 — Form 941, DE 9/DE 9C and the CDTFA quarterly return all land there, because October 31 is a Saturday
Sources & further reading

This article is general information, not tax advice for your specific situation. Rules change and details matter — talk to a CPA (we know one) before acting on anything here.